Method of producing surface-treated steel sheet

ABSTRACT

A method of producing a surface-treated steel sheet is provided. The method has a step of performing electrolytic treatment thereby to form a layer including a metal-oxygen compound on a surface of a steel sheet. The electrolytic treatment is performed so that the steel sheet is continuously fed into electrolytic treatment baths each including a treatment liquid and an electrode. The treatment liquid contains metal ions. The method is characterized in that: rolls provided for each of the electrolytic treatment baths (a roll for feeding the steel sheet into the electrolytic treatment bath and a roll for feeding the steel sheet out of the electrolytic treatment bath) comprise an energized roll that is electrically connected to a power source for flowing a direct current through the steel sheet and a non-energized roll that is not connected to a power source; and the energized roll is arranged such that, once the metal-oxygen compound has been formed by electrolytic treatment on a surface of the steel sheet to be in contact with the rolls, the metal-oxygen compound and the energized roll are not in contact with each other so as not to generate an arc spot.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a method of producing a surface-treated steel sheet.

2. Description of the Related Art

A method using chemical conversion treatment is widely employed as a method of forming a layer of which the main constituent is a metal-oxygen compound of metal, such as Zr, Al and Ti, on a metal base material. In the method using chemical conversion treatment, a metal-oxygen compound layer is formed on a metal base material such that: the metal base material is dipped in a treatment liquid; etching treatment is performed for a surface of the metal bate material; the pH in the vicinity of the surface of the metal base material is increased thereby to deposit a metal-oxygen compound on the surface of the metal base material.

According to such a method using chemical conversion, however, the rate of forming the metal-oxygen compound layer depends on the chemical reaction rate in the treatment liquid. Therefore, a long period of time is required for the metal-oxygen compound layer to be formed on the metal base material, which may be problematic.

To overcome such a problem, for example, Patent Document 1 (Japanese Patent Application Publication No. 2010-121218) discloses a method of forming a metal-oxygen compound layer by means of cathode electrolytic treatment, i.e., a method in which: an electrolytic treatment liquid that contains a metal-oxygen compound is used; hydrogen is generated in the vicinity of the surface of the metal base material by means of electrolysis of water contained in the electrolytic treatment liquid so that the pH in the vicinity of the surface of the metal base material is increased; and the metal-oxygen compound is thereby deposited on the metal base material. Using this cathode electrolytic treatment disclosed in Patent

Document 1 allows the metal-oxygen compound layer to be formed in a shorter period of time than that in the method using chemical conversion treatment.

SUMMARY OF THE INVENTION

However, the above cathode electrolytic treatment as disclosed in Patent Document 1 employs a configuration comprising electrolytic treatment baths, and all of the rolls, i.e., rolls for feeding the metal base material into the electrolytic treatment baths and rolls for feeding the metal base material out of the electrolytic treatment baths, are made as energized rolls (conductor rolls) that are electrically connected to a power source. Such energized rolls are used to energize the metal base material to perform the cathode electrolytic treatment. Therefore, problems will arise as below.

In the above cathode electrolytic treatment as disclosed in Patent Document 1, the metal-oxygen compound layer formed by the electrolytic treatment is an insulating layer, and therefore, an excessive voltage is applied from the energized rolls used for feeding the metal base material formed with the metal-oxygen compound layer out of the electrolytic treatment baths to an energized roll for energizing the metal base material. In this case, debris, such as released substances of the metal-oxygen compound layer formed on the metal base material and a precipitate of the electrolytic treatment liquid, may be deposited on the energized roll.

Since an excessive voltage is being applied to the energized roll as described above, irregularities due to such deposited substances cause high voltage discharge to locally take place on the energized roll, which may be problematic because the appearance failure such as a trace of discharge (arc spot) occurs on the metal base material. In addition, the cathode electrolytic treatment as disclosed in Patent Document 1 involves a problem in that the energized roll may be damaged by being applied with an excessive voltage and a problem in that, when the high voltage discharge locally takes place, maintenance of the energized roll will be needed in each case. Moreover, if a steel sheet formed with an arc spot is used for metal cans or other sensitive products, the arc spot portion will be a defect portion in the surface treatment thereby to induce corrosion resistance failure inside the cans and appearance failure outside the cans. Furthermore, if the arc spot is serious, the can will get a hole, which may lead to leakage of the contents.

In particular, as the thickness of the metal-oxygen compound layer formed on the metal base material increases, the insulating ability thereof is enhanced and such a voltage applied to the energized roll tends to be high. Therefore, as the metal-oxygen compound layer is formed to have a larger thickness, such problems tend to be serious. It has thus been difficult to increase the thickness of the metal-oxygen compound layer without causing such problems as described above.

The present invention has been created in view of such actual circumstances, and an object of the present invention is to provide a method of producing a surface-treated steel sheet which can increase the thickness of a metal-oxygen compound layer formed on a metal base material and improve the productivity of a surface-treated steel sheet to be produced while preventing occurrence of appearance failure, such as an arc spot, on the metal base material.

As a result of intensive studies to achieve the above object, the present inventors have found that the above object can be achieved, when forming a layer including a metal-oxygen compound on a steel sheet by means of cathode electrolytic treatment in an electrolytic treatment bath comprising at least one electrode and a treatment liquid that contains metal ions, by making a roll for feeding the steel sheet into the electrolytic treatment bath as an energized roll that is electrically connected to a power source for flowing a direct current through the steel sheet while making a roll for feeding the steel sheet out of the electrolytic treatment bath as a non-energized roll that is not connected to a power source. The inventors have thus accomplished the present invention.

That is, according to an aspect of the present invention, there is provided a method of producing a surface-treated steel sheet. The method comprises a step of performing electrolytic treatment using an electrolytic treatment apparatus thereby to form a layer including a metal-oxygen compound on a surface of a steel sheet. The electrolytic treatment apparatus has electrolytic treatment baths each comprising a treatment liquid and at least one electrode. The treatment liquid contains metal ions. The electrolytic treatment is performed in each of the electrolytic treatment baths so that the steel sheet is continuously fed into each of the electrolytic treatment baths and a direct current flows between the steel sheet and the electrode. The method has features as below. The steel sheet is continuously fed by rolls into each of the electrolytic treatment baths that constitutes the electrolytic treatment apparatus. The rolls comprise a roll for feeding the steel sheet into the electrolytic treatment bath and a roll for feeding the steel sheet out of the electrolytic treatment bath. Both of the rolls are provided for each of the electrolytic treatment baths. The rolls provided in the electrolytic treatment apparatus comprise an energized roll that is electrically connected to a power source for flowing a direct current through the steel sheet and a non-energized roll that is not connected to a power source. The energized roll is arranged such that, once the metal-oxygen compound has been formed by electrolytic treatment on a surface of the steel sheet to be in contact with the rolls, the metal-oxygen compound and the energized roll are not in contact with each other so that an arc spot is prevented from occurring.

Among rolls provided in the electrolytic treatment apparatus, a roll for feeding the steel sheet into an electrolytic treatment bath that performs electrolytic treatment first for the surface of the steel sheet to be in contact with the rolls may be the energized roll that is electrically connected to the power source for flowing the direct current through the steel sheet.

According to another aspect of the present invention there is provided a method of producing a surface-treated steel sheet. The method comprises a step of performing electrolytic treatment using an electrolytic treatment apparatus thereby to form a layer including a metal-oxygen compound on a surface of a steel sheet. The electrolytic treatment apparatus has electrolytic treatment baths each comprising a treatment liquid and an electrode. The treatment liquid contains metal ions. The electrolytic treatment is performed in each of the electrolytic treatment baths so that the steel sheet is continuously fed into each of the electrolytic treatment baths and a direct current flows between the steel sheet and the electrode. The method has features as below. The steel sheet is continuously fed by rolls into each of the electrolytic treatment baths that constitutes the electrolytic treatment apparatus. The rolls comprise a roll for feeding the steel sheet into the electrolytic treatment bath and a roll for feeding the steel sheet out of the electrolytic treatment bath. Both of the rolls are provided for each of the electrolytic treatment baths. The rolls provided in the electrolytic treatment apparatus comprise an energized roll that is electrically connected to a power source for flowing a direct current through the steel sheet and a non-energized roll that is not connected to a power source. By adjusting combination of a resistance value of the layer including the metal-oxygen compound on a surface of the steel sheet to be in contact with the energized roll and a voltage applied to the energized roll, an arc spot is prevented from occurring when the steel sheet and the energized roll are in contact with each other.

According to still another aspect of the present invention there is provided a method of producing a surface-treated steel sheet. The method comprises a step of performing electrolytic treatment using an electrolytic treatment apparatus thereby to form a layer including a metal-oxygen compound on a surface of a steel sheet. The electrolytic treatment apparatus has electrolytic treatment baths each comprising a treatment liquid and at least one electrode. The treatment liquid contains metal ions. The electrolytic treatment is performed in each of the electrolytic treatment baths so that the steel sheet is continuously fed into each of the electrolytic treatment baths and a direct current flows between the steel sheet and the electrode. The method has features as below. The steel sheet is continuously fed by rolls into each of the electrolytic treatment baths that constitutes the electrolytic treatment apparatus. The rolls comprise a roll for feeding the steel sheet into the electrolytic treatment bath and a roll for feeding the steel sheet out of the electrolytic treatment bath. Both of the rolls are provided for each of the electrolytic treatment baths. The rolls provided in the electrolytic treatment apparatus comprise at least one energized roll that is electrically connected to a power source for flowing a direct current through the steel sheet and a non-energized roll that is not connected to a power source. By adjusting combination of a resistance value of the layer including the metal-oxygen compound on a surface of the steel sheet to be in contact with the at least one energized roll, a voltage applied to the at least one energized roll, and the number of the at least one energized roll to be provided, an arc spot is prevented from occurring when the steel sheet and the at least one energized roll are in contact with each other.

According to a further aspect of the present invention, there is provided a method of producing a surface-treated steel sheet. The method comprises a step of performing electrolytic treatment in an electrolytic treatment bath thereby to form a layer including a metal-oxygen compound on a surface of a steel sheet. The electrolytic treatment bath comprises a treatment liquid and an electrode. The treatment liquid contains metal ions.

The electrolytic treatment is performed in the electrolytic treatment bath so that the steel sheet is continuously fed into the electrolytic treatment bath and a direct current flows between the steel sheet and the electrode. The method has features as below. The steel sheet is continuously fed into the electrolytic treatment bath by a first roll for feeding the steel sheet into the electrolytic treatment bath and a second roll for feeding the steel sheet out of the electrolytic treatment bath. The first roll is an energized roll that is electrically connected to a power source for flowing a direct current through the steel sheet. The second roll is a non-energized roll that is not connected to a power source.

In the producing method of the present invention, it is preferred that the treatment liquid contains ions of at least one kind of metal selected from Zr, Al and Ti.

In the producing method of the present invention, it is preferred that pH of the treatment liquid is 2 to 5.

In the producing method of the present invention, a surface of the layer formed on the surface of the steel sheet preferably has an electrical resistance value of 0.1 Ω or more, and more preferably 0.3 Ω or more.

In the producing method of the present invention, a molar amount of metal in the layer formed on the surface of the steel sheet is preferably 0.5 mmol/m² or more, and more preferably 0.7 mmol/m² or more.

In the producing method of the present invention, the layer formed on the surface of the steel sheet preferably has a thickness of 15 nm or more.

According to a still further aspect of the present invention, there is provided a surface-treated steel sheet for metal cans produced using the above producing method.

According to the present invention, there can be provided a method of producing a surface-treated steel sheet which can increase the thickness of the metal-oxygen compound layer formed on the metal base material and improve the productivity of the surface-treated steel sheet to be produced while preventing occurrence of appearance failure, such as an arc spot, on the metal base material.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating an example of a configuration of a surface treatment apparatus according to the present embodiment.

FIG. 2 is a view illustrating an example of a configuration of electrolytic treatment baths according to the present embodiment.

FIG. 3 is a view illustrating an example of a configuration of electrolytic treatment baths according to the prior art.

FIG. 4 is a view illustrating another example of a configuration of electrolytic treatment baths according to the present embodiment.

FIG. 5 is a view illustrating still another example of a configuration of electrolytic treatment baths according to the present embodiment.

FIG. 6 is a view illustrating another example of a configuration of a surface treatment apparatus according to the present embodiment.

FIG. 7 is a view illustrating still another example of a configuration of a surface treatment apparatus according to the present embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention will hereinafter be described with reference to the drawings.

FIG. 1 is a view illustrating a configuration of a surface treatment apparatus 100 to be used in the producing method of the present embodiment. The surface treatment apparatus 100 of the present embodiment is a apparatus for forming metal-oxygen compound layers on a base material 1, and comprises, as illustrated in FIG. 1, an acid pickleing treatment bath 10, an acid pickleing liquid rinsing treatment bath 20, a first electrolytic treatment bath 30, a second electrolytic treatment bath 40, an electrolytic liquid rinsing treatment bath 50, carrier rolls 61, 63, 65, 67, 69 and 71, and sink rolls 62, 64, 66, 68 and 70. Among these carrier rolls, the carrier roll 65 to be used when the base material 1 is carried into the first electrolytic treatment bath 30 is connected electrically to an external power source via rectifiers to be described later, and a current thereby flows through the carrier roll 65. Therefore, the carrier roll 65 has a function as a conductor roll that can energize the base material 1 while carrying the base material 1. Anodes 80 a to 80 h illustrated in FIG. 1 are also connected electrically to the external power source via rectifiers, and currents thereby flow through the anodes 80 a to 80 h. Therefore, the anodes 80 a to 80 h act as electrodes when electrolytic treatment is performed for the base material 1 in the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40.

According to the present embodiment, the base material 1 is fed, in the surface treatment apparatus 100, by each carrier roll into each of the acid pickleing treatment bath 10, the acid pickleing liquid rinsing treatment bath 20, the first electrolytic treatment bath 30, the second electrolytic treatment bath 40 and the electrolytic liquid rinsing treatment bath 50 in this order, and each treatment is performed in each treatment bath. Specifically, the base material 1 is first pickleed with acid pickleing liquid in the acid pickleing treatment bath 10, and then washed with water in the acid pickleing liquid rinsing treatment bath 20 so that the acid pickleing liquid remaining on the base material 1 is washed away. Subsequently, when the base material 1 faces the anodes 80 a to 80 h in the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 which are filled with electrolytic treatment liquid, the electrolytic treatment is performed due to an action of the direct current applied from the power source via the carrier roll 65 through which the current flows, and metal-oxygen compound layers are formed on the surfaces of the base material 1. Thereafter, the base material 1 is washed with water in the electrolytic liquid rinsing treatment bath 50 so that the electrolytic treatment liquid remaining on the base material 1 is washed away.

The base material 1 is not particularly limited. For example, there can be used a hot-rolled steel sheet such as based on an aluminum-killed steel continuously cast material, a cold-rolled steel sheet obtained by cold-rolling the hot-rolled steel sheet after performing acid pickleing to remove scales (oxidized layers) on the surfaces, and a steel sheet that comprises the hot-rolled or cold-rolled steel sheet and a plated layer thereon including metal, such as Zn, Sn, Ni, Cu and Al. Such a steel sheet may be used as the base material 1 after being degreased and washed with water.

The acid pickleing treatment bath 10, filled with the acid pickleing treatment liquid, is a treatment bath for performing the acid pickleing treatment to the base material 1 as a pretreatment. The base material 1 is fed into the acid pickleing treatment bath 10 by the carrier roll 61 and dipped in the acid pickleing liquid, which removes scales (oxidized layers) on the surfaces of the base material 1. The acid pickleing liquid is not particularly limited. The type of acid, the concentration and temperature of the acid pickleing liquid and other factors can be appropriately selected depending on the type of the base material 1.

The acid pickleing liquid rinsing treatment bath 20 is a treatment bath for washing the base material 1 with water, and examples thereof include a bath filled with water. When a bath filled with water is used as the acid pickleing liquid rinsing treatment bath 20, the base material 1 is fed into the acid pickleing liquid rinsing treatment bath 20 by the carrier roll 63 and dipped in the water, which washes away the acid pickleing liquid remaining on the surfaces of the base material 1. Another example of the acid pickleing liquid rinsing treatment bath 20 may be equipment that sprays water to the base material 1 to wash the base material 1 with the water. In this case, the acid pickleing liquid remaining on the surfaces of the base material 1 can be washed away by the sprayed water without filling the acid pickleing liquid rinsing treatment bath 20 with water.

The first electrolytic treatment bath 30 and the second electrolytic treatment bath 40, filled with the electrolytic treatment liquid, are baths for forming metal-oxygen compound layers on the base material 1 by means of electrolytic treatment. The electrolytic treatment is performed in the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 as follows. The base material 1 is first fed into the first electrolytic treatment bath 30 by the carrier roll 65, and the electrolytic treatment is performed for the base material 1 due to actions of the anodes 80 a to 80 d. The base material 1 is then fed out of the first electrolytic treatment bath 30 and fed into the second electrolytic treatment bath 40 by the carrier roll 67, and the electrolytic treatment is performed likewise in the electrolytic treatment liquid for the base material 1 due to actions of the anodes 80 e to 80 h. Detailed features of the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 will be described later.

The electrolytic liquid rinsing treatment bath 50, which is a treatment bath for washing the base material 1 with water, may be filled with water or may otherwise be configured such that a spraying apparatus sprays water to the base material 1. The base material 1 is fed by the carrier roll 69 into the electrolytic liquid rinsing treatment bath 50, which washes away the electrolytic treatment liquid remaining on the surfaces of the base material 1 due to passing through the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40. In an alternative embodiment, the electrolytic liquid rinsing treatment bath 50 may comprise baths so that the former-stage bath or baths are responsible for cleaning by the electrolytic treatment liquid while the latter-stage bath or baths are responsible for washing with water. The electrolytic treatment liquid or water to be used in the electrolytic liquid rinsing treatment bath 50 may be used in a form in which the electrolytic liquid rinsing treatment bath 50 is filled with the electrolytic treatment liquid or water and the base material 1 is dipped therein, or may otherwise be sprayed to the base material 1 using a spraying apparatus provided in the electrolytic liquid rinsing treatment bath 50. The dipping into or the spraying of the electrolytic treatment liquid before the washing with water can remove unnecessary layers formed on the base material 1. In an alternative embodiment, an additional electrolytic liquid rinsing treatment bath 50 may be provided between the first electrolytic treatment bath 30 as the first stage and the second electrolytic treatment bath 40 as the last stage, both for performing the electrolytic treatment.

FIG. 2 is a view illustrating in detail the features of the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 illustrated in FIG. 1. As illustrated in FIG. 2, the first electrolytic treatment bath 30 is filled with electrolytic treatment liquid 31, in which four anodes 80 a to 80 d are dipped. In addition, the carrier rolls 65 and 67 and the sink roll 66 are provided respectively around and in the first electrolytic treatment bath 30.

The carrier roll 65 is a roll for feeding the base material 1 out of the above-described acid pickleing liquid rinsing treatment bath 20 and feeding the base material 1 into the first electrolytic treatment bath 30. The carrier roll 65 is connected electrically to an external power source via rectifiers 90 to be described later, and has a function as a conductor roll that can energize the base material 1 while carrying the base material 1. The sink roll 66 is a roll for turning the traveling direction of the base material 1 in the electrolytic treatment liquid. The carrier roll 67 is a roll for feeding the base material 1 out of the first electrolytic treatment bath 30 and feeding the base material 1 into the second electrolytic treatment bath 40. It should be emphasized that, unlike the above-described carrier roll 65, the carrier roll 67 is a non-energized roll that is not connected to a power source.

Like the first electrolytic treatment bath 30, the second electrolytic treatment bath 40 is also filled with electrolytic treatment liquid 41, in which the anodes 80 e to 80 h are dipped.

The carrier rolls 67 and 69 and the sink roll 68 are provided respectively around and in the second electrolytic treatment bath 40. Like the carrier roll 67, the carrier roll 69 is also a non-energized roll that is not connected to a power source.

Rectifiers 90 are provided outside the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40, and are connected to an external power source (not shown). The rectifiers 90 are electrically connected to respective anodes 80 a to 80 h which are dipped in the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40. This allows a current to flow through each anode, which therefore acts as an oxidation electrode (electrode at which electrons are extracted) for the base material 1 when the electrolytic treatment is performed.

All of the rectifiers 90 connected to the anodes are also connected electrically to the carrier roll 65. This allows a current to flow through the carrier roll 65, which therefore acts as a conductor roll that can cause the current to flow through the base material 1 while carrying the base material 1. Thus, the carrier roll 65 energizes the base material 1, which is fed in the energized state into the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 by each carrier roll, so that the electrolytic treatment is performed due to actions of the anodes 80 a to 80 h to form the metal-oxygen compound layers on the base material 1.

It is preferred to use, as the material for the anodes 80 a to 80 h, an insoluble metal such as platinum and stainless steel or a coating metal such as titanium deposited thereon with iridium oxide because they have high electrochemical stability. The rectifiers 90 are not particularly limited. Rectifiers known in the art can be used depending on the magnitude of electrical power supplied to the carrier roll 65 and the anodes 80 a to 80 h.

The electrolytic treatment liquid 31 and the electrolytic treatment liquid 41 are each aqueous solution that contains metal ions for forming the metal-oxygen compound layers on the base material 1. Here, it is preferred that the metal ions contained in the electrolytic treatment liquid 31 and the electrolytic treatment liquid 41 are ions of at least one kind of metal selected from Zr, Al and Ti because they can well form the metal-oxygen compound layers on the base material 1, among which Zr is particularly preferred. When such electrolytic treatment liquid is continuously used in the electrolytic treatment, an amount of impurities increases in the electrolytic treatment liquid, and the efficiency in the electrolytic treatment and the quality of products will deteriorate. Therefore, the electrolytic treatment may be performed while new electrolytic treatment liquid is appropriately circulated in the electrolytic treatment bath or baths. For example, after preliminarily preparing a larger amount of the electrolytic treatment liquid 31 than the volume of the first electrolytic treatment bath 30 and storing a part of the prepared electrolytic treatment liquid 31 in a treatment liquid bath (not shown) provided outside the first electrolytic treatment bath 30, the electrolytic treatment may be performed while circulating the liquid between the treatment liquid bath and the first electrolytic treatment bath 30 using a pump or other appropriate means. The second electrolytic treatment bath 40 may also be configured to circulate the electrolytic treatment liquid 41 in a similar manner.

According to the present embodiment, the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 are used to perform the electrolytic treatment for the base material 1 to form the metal-oxygen compound layers on the base material 1, as will be described below.

First, the base material 1 is fed into the first electrolytic treatment bath 30 by the carrier roll 65, and carried through between the anodes 80 a and 80 b dipped in the electrolytic treatment liquid 31 in the first electrolytic treatment bath 30. The base material 1 faces the anodes 80 a and 80 b when passing through between the anodes 80 a and 80 b, and cathode electrolytic treatment is performed due to the action of the direct current applied from the power source via the carrier roll 65 through which the current flows, so that the metal-oxygen compound layers are formed on the surfaces of the base material 1.

Specifically in the cathode electrolytic treatment, currents flow between the base material 1 and the anodes 80 a and 80 b to generate hydrogen in the vicinity of the surfaces of the base material 1 due to electrolysis of water in the electrolytic treatment liquid 31. This increases the pH in the vicinity of the surfaces of the base material 1, and the increased pH causes metal ions contained in the electrolytic treatment liquid 31 to be deposited as an oxygen compound. The metal-oxygen compound layers are thus formed on the base material 1. For example, when the electrolytic treatment liquids 31 and 41 contain Zr ions, metal-oxygen compound layers that contain an oxygen compound of Zr are formed on the base material 1. In a similar manner, when the electrolytic treatment liquids 31 and 41 contain Al ions, for example, metal-oxygen compound layers that contain an oxygen compound of Al are formed on the base material 1. When the electrolytic treatment liquids 31 and 41 contain Ti ions, metal-oxygen compound layers that contain an oxygen compound of Ti are formed on the base material 1.

After the cathode electrolytic treatment is performed due to actions of the anodes 80 a and 80 b, the sink roll 66 turns the traveling direction of the base material 1, which then faces the anodes 80 c and 80 d in the electrolytic treatment liquid 31, so that the cathode electrolytic treatment is performed again to further form metal-oxygen compound layers on the base material 1. The base material 1 is then fed out of the first electrolytic treatment bath 30 and fed into the second electrolytic treatment bath 40 by the carrier roll 67. In the electrolytic treatment liquid 41 in the second electrolytic treatment bath 40, the cathode electrolytic treatment is performed in a similar manner due to actions of the anodes 80 e and 80 f and subsequently due to actions of the anodes 80 g and 80 h, thereby to further form metal-oxygen compound layers on the base material 1. Thereafter, the base material 1 is fed out of the second electrolytic treatment bath 40 by the carrier roll 69. According to the present embodiment, the electrolytic treatment is thus performed for the base material 1 using the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40.

According to the present embodiment, as illustrated in FIG. 2, the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 are employed to form the metal-oxygen compound layers on the base material 1 so that the carrier roll 65 is used as an energized conductor roll while the carrier rolls 67 and 69 are used as non-energized rolls. Therefore, when the metal-oxygen compound layers are formed on the base material 1, the voltage applied to the carrier roll can be suppressed to prevent the occurrence of appearance failure, such as a trace of discharge (arc spot), which occurs on the base material 1, as will be described below. The present embodiment is described as an example in which two electrolytic treatment baths are used, but the advantageous effects of the present invention can also be obtained when three or more electrolytic treatment baths are used for the purpose of obtaining a necessary amount of the layers.

First, the metal-oxygen compound layers formed by the electrolytic treatment are insulating layers, and therefore, when an energized conductor roll is used to flow a direct current through the base material 1 formed with a large thickness of the metal-oxygen compound layers, an excessive voltage is applied to the conductor roll. In this case, debris, such as released substances of the metal-oxygen compound layers formed on the base material 1 and a precipitate of the electrolytic treatment liquid, may be deposited on the conductor roll, and if an excessive voltage is being applied to the conductor roll, irregularities due to such deposited substances cause high voltage discharge to locally take place on the conductor roll, which may be problematic because the appearance failure such as an arc spot occurs on the base material 1.

Here, the carrier rolls 67 and 69 are rolls for carrying the base material 1 formed with the metal-oxygen compound layers, and if the carrier rolls 67 and 69 are energized conductor rolls, excessive voltages will be applied to the carrier rolls 67 and 69 to possibly generate an arc spot on the base material 1. In particular, the carrier roll 69 is a roll for carrying the base material 1 formed with a larger thickness of the metal-oxygen compound layers compared to the carrier roll 67. To energize the base material 1 via such a larger thickness of the metal-oxygen compound layer, a higher voltage may have to be applied to the carrier roll 69, on which the problem of an arc spot on the base material 1 will be serious.

In contrast, according to the present embodiment, no currents flow through the carrier rolls 67 and 69, and excessive voltages are not applied to the carrier rolls 67 and 69 even when they carry the base material 1 formed with the metal-oxygen compound layers. Therefore, high voltage discharge does not locally take place and the occurrence of an arc spot on the base material 1 can be effectively prevented even when irregularities are formed on the surfaces of the carrier rolls 67 and 69 such as due to deposition of the metal-oxygen compound layers.

As a conventional method of forming metal-oxygen compound layers on the base material 1 by means of electrolytic treatment, there has been used a method in which, as illustrated in FIG. 3, all the carrier rolls provided around the electrolytic treatment baths are made as energized conductor rolls to perform the electrolytic treatment. The first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 illustrated in FIG. 3 comprise electrolytic treatment liquids 31 and 41, sink rolls 66 and 68, anodes 80 a to 80 h and rectifiers 90 as with the case of FIG. 2 except that carrier rolls 65, 67 a and 69 a are all energized conductor rolls.

In such a configuration as illustrated in FIG. 3, when the carrier rolls 67 a and 69 a energize the base material 1, excessive voltages are applied to the carrier rolls 67 a and 69 a as described above, and an arc spot may possibly occur on the base material 1 if irregularities are formed on the surfaces of the carrier rolls 67 a and 69 a such as due to deposition of the metal-oxygen compound layers. In particular, the carrier roll 69 a is a roll for carrying the base material 1 formed with a larger thickness of the metal-oxygen compound layers compared to the carrier roll 67 a. When energizing the base material 1 from the carrier roll 69 a, therefore, a further excessive voltage will be applied to the carrier roll 69 a, on which such a problem of an arc spot may be serious. In addition, the carrier roll 69 a involves a problem in that it may be damaged by being applied with an excessive voltage and a problem in that, when the high voltage discharge locally takes place, maintenance will be needed in each case. Conventionally, such problems regarding the energizing method have not occurred because the metal-oxygen compound layers are contemplated within a range of a small amount of the layers. However, depending on the intended use of the surface-treated steel sheet, such as in a case which requires an enhanced property of resistance to the contents of metal cans, a larger amount of the layers has been needed. In this case, the above problems regarding the energizing method may be serious, which will be problematic in the productivity and quality.

The rate at which the electrolytic treatment forms the metal-oxygen compound layers depends on the rate of rise of the pH in the vicinity of the base material 1 when hydrogen is generated due to electrolysis of water in the electrolytic treatment liquid in the vicinity of the base material 1. Therefore, the above problems will be serious as the rate of forming the metal-oxygen compound layers is increased and as the metal-oxygen compound layers are tried to be made with an increased thickness because, in order to increase the rate of forming the metal-oxygen compound layers on the base material 1, high voltages have to be applied to the rolls for causing electrolysis of water.

In contrast, according to the present embodiment, the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 are employed, as illustrated in FIG. 2, to form the metal-oxygen compound layers on the base material 1 so that the carrier roll 65 is used as an energized conductor roll while the carrier rolls 67 and 69 are used as non-energized rolls.

In particular, according to the present embodiment, since the carrier roll 65 is used as an energized conductor roll while the carrier rolls 67 and 69 are used as non-energized rolls, excessive voltages are not applied to the carrier rolls 67 and 69 even when the carrier rolls 67 and 69 carry the base material 1 formed with the metal-oxygen compound layers. Therefore, high voltage discharge does not locally take place on the carrier rolls 67 and 69, and the occurrence of an arc spot on the base material 1 can be effectively prevented, even when irregularities are formed on the surfaces of the carrier rolls 67 and 69 such as due to deposition of the metal-oxygen compound layers.

In the above manner, according to the producing method of the present embodiment, it is possible to increase the thickness of the metal-oxygen compound layers formed on the base material 1 while preventing the occurrence of appearance failure, such as an arc spot, on the base material 1. Moreover, according to the producing method of the present embodiment, the productivity of the surface-treated steel sheet to be produced can be improved because excessive voltages are not applied to the carrier rolls so that the carrier rolls are prevented from being damaged and the frequency of maintenance of the carrier rolls can be reduced.

When the surface-treated steel sheet produced by forming the metal-oxygen compound layers on the base material 1 is used as a material for seamless cans or three-piece cans, an organic resin layer is further formed on the surface-treated steel sheet, which is then formed into a form of cans. When seamless cans or three-piece cans are produced, the surface-treated steel sheet undergoes various forces, such as tension and compression, through drawing process, drawing and ironing process, necking process, flanging process or other processes. Even after being produced, the cans may be placed in a severe environment for the metal surface, such as when the cans are filled with food or beverage and then undergo retort treatment or other heat treatment and when the cans are filled with coffee or other contents and stored in a state of being heated at a vendor. Therefore, when the surface-treated steel sheet is used for cans, it is necessary to avoid cracks in the metal-oxygen compound layers and suppress exposure of the steel sheet of the base material 1 as much as possible. If the exposure of the steel sheet of the base material 1 is not sufficiently suppressed, the organic resin layer of seamless cans or three-piece cans is likely to delaminate from a part at which the steel sheet of the base material 1 is exposed, and the resistance to the contents of cans (corrosion resistance) will deteriorate and/or poor quality of printing appearance may be induced at the side of outer surface of the cans. In particular, if the exposure of the steel sheet of the base material 1 is not sufficiently suppressed, the cans stored after being filled with the contents and undergoing the retort sterilization treatment may be deformed such as due to drop impact, so that the organic resin layer is likely to delaminate when the cans are further stored for a period of time, and there may be significantly recognized the deterioration in the resistance to the contents of cans (corrosion resistance) and poor quality of printing appearance at the side of outer surface of the cans.

In such situations, according to the producing method of the present embodiment, the metal-oxygen compound layers can be formed with a large layer thickness on the base material 1. Therefore, even when the surface-treated steel sheet thus obtained is used for producing seamless cans or three-piece cans, it is possible to suppress exposure of the base material 1 and produce cans that are excellent in the resistance to the contents of cans (corrosion resistance) and the printing appearance.

Thus, according to the present embodiment, the metal-oxygen compound layers formed on the base material 1 can have a large layer thickness preferably of 15 nm or more, and more preferably of 25 nm or more. This allows imparting excellent resistance to the contents of cans (corrosion resistance) and printing appearance when the obtained surface-treated steel sheet is applied to metal cans.

The amount of layers suitable for the metal-oxygen compound layers may preferably be 0.5 mmol/m² or more, and more preferably 0.7 mmol/m² or more, as a molar amount of the metal contained in the metal-oxygen compound layers. For example, the amount of the metal-oxygen compound layers may preferably be about 46 mg/m² or more, and more preferably about 64 mg/m² or more, as a weight layer thickness when the metal-oxygen compound consists only of Zr compound, or may preferably be about 14 mg/m² or more, and more preferably about 19 mg/m² or more, as a weight layer thickness when the metal-oxygen compound consists only of Al compound, or may preferably be about 24 mg/m² or more, and more preferably about 34 mg/m² or more, as a weight layer thickness when the metal-oxygen compound consists only of Ti compound. As will be understood, the metal-oxygen compound may be a mixture of two or more kinds selected from Zr compound, Al compound and Ti compound.

The molar amount of the metal contained in the metal-oxygen compound layers is thus preferably 0.5 mmol/m² or more, and more preferably 0.7 mmol/m² or more. This allows imparting excellent resistance to the contents of cans (corrosion resistance) and printing appearance when the obtained surface-treated steel sheet is applied to metal cans.

The electrical resistance suitable for the metal-oxygen compound layers may preferably be 0.1 Ω or more, and more preferably 0.3 Ω or more. The electrical resistance at the surface of the metal-oxygen compound layer within the above range allows the obtained surface-treated steel sheet to have excellent insulation property and also allows imparting excellent resistance to the contents of cans (corrosion resistance) and printing appearance when the surface-treated steel sheet is applied to metal cans.

The above-described embodiment exemplifies a configuration in which the carrier roll 65 is used as an energized conductor roll while the carrier rolls 67 and 69 are used as non-energized rolls. In an alternative embodiment, as illustrated in FIG. 4, for example, the carrier roll, which is a roll for feeding the base material 1 out of the first electrolytic treatment bath 30 and carrying the base material 1 into the second electrolytic treatment bath 40, may be an energized conductor roll (denoted by a reference “67 a” in FIG. 4) as with the carrier roll 65. When electrolytic treatment baths are used, if only one carrier roll 65 is employed as an energized conductor roll, total current necessary for the electrolytic treatment is to flow through the carrier roll 65. Unduly high electrical current leads to surface defects on the surface-treated steel sheet because an arc spot may occur even on the carrier roll 65 and the plating on the roll surface may delaminate, such as due to foreign materials carried in from the previous process. Accordingly, the carrier roll 67 a may be made as an energized conductor roll, if necessary, thereby to avoid such problems. In this case, the value of a current that flows through the carrier roll 67 a may be adjusted in accordance with actual operation. For example, the amount of the metal-oxygen compound layers formed in accordance with the current value, i.e., the amount of the metal-oxygen compound layers formed in accordance with the value of a current that flows through the carrier roll 67 a into the anodes 80 a and 80 b, may be to such an extent that an arc spot does not occur when the metal-oxygen compound layer comes into contact with the carrier roll 67 a. In an alternative embodiment, spray equipment or drawing rolls may be provided between the electrolytic treatment liquid surface and the carrier roll 67 a or 69 thereby to have a role to reduce unnecessary layers formed on the base material 1 and suppress the occurrence of an arc spot.

According to the configuration illustrated in FIG. 4, the carrier rolls 65 and 67 a energize the base material 1. Here, the carrier roll 67 a is a roll for carrying the base material 1 formed with the metal-oxygen compound layers, and a high voltage is thus to be applied to the carrier roll 67 a when the base material 1 is energized via such a metal-oxygen compound layer. However, when the base material 1 reaches the carrier roll 67 a, the base material 1 is in a state before undergoing the electrolytic treatment in the second electrolytic treatment bath 40. Therefore, the metal-oxygen compound layers formed on the base material 1 are relatively thin, so that the voltage necessary for the carrier roll 67 a illustrated in FIG. 4 to energize the base material 1 is smaller than that for the above-described carrier roll 69 a illustrated in FIG. 3.

For the above reason, when the carrier rolls 65 and 67 a are used as energized conductor rolls while the carrier roll 69 is used as a non-energized roll as illustrated in FIG. 4, an excessive voltage can be prevented from being applied to the carrier roll 67 a depending on the thickness of the metal-oxygen compound layers formed on the base material 1 in the first electrolytic treatment bath 30 and also depending on the magnitude of a current that flows through the carrier roll 67 a. The voltage to be applied can thus be reduced thereby to avoid an excessive voltage from being applied to the carrier roll 67 a and prevent the occurrence of appearance failure such as an arc spot on the base material 1 due to local discharge. In this manner, the occurrence of appearance failure such as an arc spot can be effectively prevented. According to the present embodiment, therefore, even when the electrolytic treatment baths configured as illustrated in FIG. 4 are used, it is possible to increase the thickness of the metal-oxygen compound layers formed on the base material 1 and improve the productivity of the surface-treated steel sheet to be produced while preventing the occurrence of appearance failure, such as an arc spot, on the base material 1.

In a modified embodiment of the present embodiment, as illustrated in FIG. 5, two anodes may be arranged in the first electrolytic treatment bath 30 at the side facing the surface of the base material 1 which does not come into contact with the carrier roll 67 a while four anodes may ordinarily be arranged in the second electrolytic treatment bath 40. In this embodiment, the carrier rolls 65 and 67 a are used as energized conductor rolls while the carrier roll 69 is used as a non-energized roll.

In another embodiment in which the anodes are arranged like in FIG. 5, only the carrier roll 67 a may be used as an energized conductor roll while the carrier rolls 65 and 69 may be used as non-energized rolls.

In particular, as illustrated in FIG. 5, the present embodiment employs a configuration in which, after a metal-oxygen compound layer is once formed on the surface of the base material 1 which comes into contact with each carrier roll (the surface will be referred hereinafter to as a “carrier roll-side surface”), the metal-oxygen compound layer does not come into contact with an energized conductor roll. This allows appropriately preventing the occurrence of appearance failure such as an arc spot. More specifically, in the configuration as illustrated in FIG. 5, the anodes 80 b and 80 c are arranged at the opposite side to the carrier roll-side surface of the base material 1. Therefore, when the base material 1 reaches the carrier roll 67 a after passing through the first electrolytic treatment bath 30, the surface of the base material 1 to come into contact with the carrier roll 67 a is formed thereon with a relatively small amount of the metal-oxygen compound layer. It is thereby possible to prevent an excessive voltage from being applied to the carrier roll 67 a and appropriately prevent the occurrence of appearance failure such as an arc spot.

In the configuration as illustrated in FIG. 5, the carrier roll 67 a, which is for feeding the base material 1 into the electrolytic treatment bath (second electrolytic treatment bath 40) that performs the electrolytic treatment first for the carrier roll-side surface of the base material 1, acts as an energized conductor roll. Therefore, a current flows well through the base material 1 in the second electrolytic treatment bath 40 from the carrier roll 67 a adjacent to the second electrolytic treatment bath 40, and the electrolytic treatment can be efficiently performed in the second electrolytic treatment bath 40.

The above-described embodiment exemplifies a configuration in which two electrolytic treatment baths, i.e., the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40, are used to perform the electrolytic treatment for the base material 1. In an alternative embodiment, however, the second electrolytic treatment bath 40 may be omitted as illustrated in FIG. 6 so that the first electrolytic treatment bath 30 is solely used to perform the electrolytic treatment for the base material 1. In this embodiment, the configuration may be modified as illustrated in FIG. 7 in which the number of anodes provided in the first electrolytic treatment bath 30 is reduced to two.

While the above-described surface treatment apparatus 100 as illustrated in FIG. 1 includes two electrolytic treatment baths, i.e., the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40, the number of electrolytic treatment baths provided in the surface treatment apparatus 100 is not particularly limited, and three or more electrolytic treatment baths may also be used. Likewise, while the surface treatment apparatus 100 as illustrated in FIG. 1 includes one acid pickleing treatment bath 10, one acid pickleing liquid rinsing treatment bath 20, and one electrolytic liquid rinsing treatment bath 50, the number of those baths is not particularly limited, and respective two or more baths may be provided.

In the present embodiment, metal compounds that constitute the electrolytic treatment liquid 31 and the electrolytic treatment liquid 41 are not particularly limited. For example, when Zr ions are to be contained in the electrolytic treatment liquid 31 and the electrolytic treatment liquid 41, appropriate Zr compounds can be used, such as K₂ZrF₆, (NH₄)₂ZrF₆, (NH₄)₂ZrO(CO₃)₂, ZrO(NO₃)₂, and ZrO(CH₃COO)₂. When Al ions are to be contained, for example, appropriate Al compounds can be used, such as Al(NO₃)₃·9H₂O, AlK(SO₄)₂·12H₂O, Al₂(SO₄)₃·13H₂O, Al(H₂PO₄)₃, AlPO₄, and [CH₃CH(OH)COO]₃Al. When Ti ions are to be contained, for example, appropriate Ti compounds can be used, such as K₂TiF₆, (NH₄)₂TiF₆, Na₂TiF₆, K₂TiO(C₂O₄)₂·2H₂O, TiCl₃, and TiCl₄. In the electrolytic treatment liquid 31 and the electrolytic treatment liquid 41 according to the present embodiment, one kind of the above-described compound or the like may be solely used, or two or more kinds may be used in combination. The electrolytic treatment liquid 31 and the electrolytic treatment liquid 41 may be the same aqueous solution, or may otherwise be different solutions prepared using different metal compounds or compounded with different compounding ratios of the same metal compound.

To enhance the solubility of ions of metal such as Zr, Al and Ti in the liquid, a complexing agent such as fluoride and cyanide may be contained in the electrolytic treatment liquid 31 and the electrolytic treatment liquid 41. The concentrations of metal ions and the complexing agent contained in the electrolytic treatment liquid 31 and the electrolytic treatment liquid 41 are not particularly limited, and may appropriately be set for adjusting the conductivity and the current density in the treatment liquid and adjusting the amount of the metal-oxygen compound layers to be formed.

To enhance the conductivity of the treatment liquid, an electrolyte such as nitrate ions and ammonium ions may be contained in the electrolytic treatment liquid 31 and the electrolytic treatment liquid 41 to such an extent that does not inhibit the formation of the metal-oxygen compound layers.

The pH of the electrolytic treatment liquid 31 and the electrolytic treatment liquid 41 may preferably be 2.0 to 5.0, and more preferably 2.5 to 4.0. Unduly low pH will cause the surfaces of the base material 1 to be excessively etched, so that the metal-oxygen compound layers may be difficult to be formed. On the other hand, unduly high pH will cause unnecessary metal-oxygen compounds to precipitate in the electrolytic treatment liquid 31 and the electrolytic treatment liquid 41, and the deposition of the metal-oxygen compound onto the base material 1 may tend to be inhibited.

Organic acid, such as polyacrylic acid, polyitaconic acid, citric acid, lactic acid, tartaric acid, glycolic acid, phenolic resin and hydroxy acid, may be added to the electrolytic treatment liquid 31 and the electrolytic treatment liquid 41. By adding such an organic acid, when organic resin layers are formed on the metal-oxygen compound layers, the interfacial adhesion can be improved between the metal-oxygen compound layers and the organic resin layers. Such organic resin layers are formed on the metal-oxygen compound layers when the surface-treated steel sheet formed with the metal-oxygen compound layers on the surfaces is used as a material for seamless cans or three-piece cans.

Each carrier roll provided in the surface treatment apparatus 100 illustrated in FIG. 1 is exemplified as one roll, but may comprise two or more rolls. For example, the carrier roll 67, which is a roll for feeding the base material 1 out of the first electrolytic treatment bath 30 and feeding the base material 1 into the second electrolytic treatment bath 40, may comprise a roll for feeding the base material 1 out of the first electrolytic treatment bath 30 and a roll for feeding the base material 1 into the second electrolytic treatment bath 40. Material of each carrier roll is not particularly limited. For the carrier rolls which act as non-energized rolls, electrically insulating material such as rubber may be used, for example.

Each carrier roll may be provided with a nip roll for holding the base material 1 when carrying the base material 1 and/or a ringer roll for removing the treatment liquid remaining on the surface of the base material 1 not facing the carrier roll to prevent the treatment liquid from being brought outside the treatment bath.

<Producing of Surface-Treated Steel Sheet for Metal Cans>

The surface-treated steel sheet produced according to the producing method of the present invention can be used as a member that constitutes a metal can (surface-treated steel sheet for metal cans). In the surface-treated steel sheet for metal cans, it is important to suppress the exposure of the steel sheet of the base material 1, as previously described.

That is, the surface-treated steel sheet for metal cans may preferably be a surface-treated steel sheet having metal-oxygen compound layers that contain ions of at least one kind of metal selected from Zr, Al and Ti, wherein:

-   1. the thickness of the layers is preferably 15 nm or more, and more     preferably 25 nm or more, and 160 nm or less; -   2. the molar amount of the metal contained in the metal-oxygen     compound layers is preferably 0.5 mmol/m² or more, and more     preferably 0.7 mmol/m² or more, and 4.4 mmol/m² or less; and -   3. the electrical resistance of the metal-oxygen compound layers is     preferably 0.1 Ω or more, and more preferably 0.3 Ω or more, and     3,500 Ω or less.

When at least one condition of the above 1 to 3 is satisfied, the obtained surface-treated steel sheet can be imparted with excellent resistance to the contents of cans (corrosion resistance) and printing appearance when the surface-treated steel sheet is applied to metal cans. Unduly large thickness of the metal-oxygen compound layers leads to deterioration in the interfacial adhesion because cracks may occur in the layers such as during the formation of cans, necking process and flanging process. Therefore, a preferred range of the layer thickness may exist.

<Producing of Organic Resin-Coated Surface-Treated Steel Sheet for Metal Cans>

When the surface-treated steel sheet produced according to the producing method of the present invention is applied to metal cans, it is preferred to form at least one organic resin layer on the surface thereof to prepare an organic resin-coated surface-treated steel sheet for metal cans which is a member obtained by coating the surface-treated steel sheet with the organic resin layer and which can be used as a member for metal cans. Examples of the organic resin layer include, but are not particularly limited to, thermoplastic resin-coating layers formed of various thermoplastic resins, and coating layers formed of thermoset coating material or thermoplastic coating material. Primer for adhesion or adhesive as known in the art may be provided between the surface-treated steel sheet and the organic resin layer when the organic resin layer is formed.

It is preferred to use, among the above organic resin layers, a thermoplastic resin-coating layer formed of polyester resin suitable for the use as a container material. The polyester resin may be homo-polyethylene terephthalate or copolymerized polyester, or may also be a mixture thereof. When the thermoplastic resin-coating layer is used as the organic resin layer, the thermoplastic resin-coating layer may be a single resin layer or a multilayer obtained such as by coextrusion. When the thermoplastic resin-coating layer is a multilayer, the resin that constitutes each sub-layer may be or may not be the same. Using a multilayer of polyester resin layers for forming the thermoplastic resin-coating layer is advantageous, because a polyester resin of a composition excellent in the interfacial adhesion can be selected as the material of an underlying layer which is located at the side of the surface-treated steel sheet, and a polyester resin of a composition excellent in the resistance to the contents of cans, i.e., resistance to extraction and non-adsorptive property for flavor constituents, can be selected as the material for a surface layer.

It is preferred that the thickness of the organic resin layer provided on the surface-treated steel sheet is ordinarily 3 to 50 μm, and particularly 5 to 40 μm, as that of the thermoplastic resin-coating layer. It is also preferred that the thickness after baking is 1 to 50 μm, and particularly 3 to 30 μm, as that of the coating layer. If the thickness is below the above range, the corrosion resistance will be insufficient, while if the thickness is above the above range, problems in workability may arise.

Formation of the organic resin layer on the surface-treated steel sheet can be performed using any method which has conventionally been known. For example, when the thermoplastic resin-coating layer is formed as the organic resin layer, there can be used extrusion coating method, cast layer thermal adhesion method, biaxially-stretched layer thermal adhesion method, or any other appropriate means.

<Producing of Metal Cans>

The organic resin-coated surface-treated steel sheet for metal cans produced using the producing method of the present invention can be suitably used for three-piece cans (welded cans) having side seams, seamless cans (two-piece cans), and other cans. The seamless cans may be produced such that the organic resin layer is located inside the can, by any conventionally known means, such as drawing process, drawing/redrawing process, stretching process via drawing/redrawing, stretching/ironing process via drawing/redrawing, or drawing/ironing process. In particular, the present invention can be most suitably applied to seamless cans, among the seamless cans produced through the above processes, which are produced using a highly sophisticated process, such as stretching process via drawing/redrawing and stretching/ironing process via drawing/redrawing.

That is, the organic resin-coated surface-treated steel sheet for metal cans is excellent in the workability and the interfacial adhesion, and there can thus be provided seamless cans that are excellent in the interfacial adhesion with the organic resin layers and have excellent corrosion resistance even after undergoing such a highly sophisticated process.

Embodiments of the present invention have been heretofore explained, but these embodiments are described to facilitate understanding of the present invention and are not described to limit the present invention. Therefore, it is intended that the elements disclosed in the above embodiments include all design changes and equivalents to fall within the technical scope of the present invention.

For example, the above-described embodiment illustrated in FIG. 5 exemplifies a configuration in which, after a metal-oxygen compound layer is once formed on the surface of the base material 1 which comes into contact with each carrier roll (carrier roll-side surface), the metal-oxygen compound layer does not come into contact with an energized conductor roll, but another configuration may be employed such that the metal-oxygen compound layer comes into contact with at least one energized conductor roll. In this configuration, the occurrence of an arc spot can be prevented by appropriately adjusting an electrical resistance value of the metal-oxygen compound layer formed on the carrier roll-side surface of the base material 1, a voltage applied to the at least one energized conductor roll, and the number of the at least one energized conductor roll to be provided.

Here, as the electrical resistance value of the metal-oxygen compound layer formed on the carrier roll-side surface of the base material 1 decreases, there is a tendency that, when the base material 1 comes into contact with a conductor roll, high voltage discharge is less likely to occur locally on the conductor roll to generate an arc spot. Similarly, as the voltage applied to a conductor roll decreases, there is a tendency that, when the base material 1 comes into contact with the conductor roll, high voltage discharge is less likely to occur locally on the conductor roll to generate an arc spot. Therefore, the present embodiment may employ a configuration in which the occurrence of an arc spot is prevented by balancing the electrical resistance value of the metal-oxygen compound layer and the voltage applied to the conductor roll.

In particular, according to the present embodiment, the electrical resistance value of the metal-oxygen compound layer depends on the thickness of the metal-oxygen compound layer formed on the base material 1 in general. Therefore, when the thickness of the metal-oxygen compound layer is relatively small, the voltage applied to the conductor roll may be adjusted relatively high, while when the thickness of the metal-oxygen compound layer is relatively large, the voltage applied to the conductor roll may be adjusted relatively low, for example. When conductor rolls are provided, the voltage applied to conductor roll or rolls to come into contact with the base material 1 formed with a relatively thin metal-oxygen compound layer may be adjusted relatively high, while the voltage applied to conductor roll or rolls to come into contact with the base material 1 formed with a relatively thick metal-oxygen compound layer may be adjusted relatively low. In this case, even when the voltages applied to the conductor rolls are adjusted, it is preferred to increase or decrease the number of energized conductor rolls so that the metal-oxygen compound layer formed on the base material 1 by means of electrolytic treatment has a desired thickness. More specifically, when the voltages applied to the conductor rolls are set low, for example, the number of energized conductor rolls may be increased accordingly, thereby to allow the metal-oxygen compound layer formed on the base material 1 by means of electrolytic treatment to have a desired thickness.

Thus, according to the present embodiment, the occurrence of an arc spot can effectively be prevented by appropriately adjusting combination of an electrical resistance value of the metal-oxygen compound layer, voltages applied to the energized conductor rolls, and the number of the energized conductor rolls to be provided.

EXAMPLES

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples.

The carrier roll 65 may be referred hereinafter to as a “first roll,” the carrier roll 67 or 67 a as a “second roll,” and the carrier roll 69 or 69 a as a “third roll.”

Example 1

A cold-rolled steel sheet (thickness of 0.2 mm and width of 200 mm) was prepared as a raw sheet.

The prepared steel sheet was electrolytically degreased and then washed with water, and thereafter the surface treatment apparatus 100 illustrated in FIG. 6 was used to perform cathode electrolytic treatment. In the cathode electrolytic treatment, pretreatment was first performed such that: the steel sheet was fed by the carrier roll 61 into the acid pickleing treatment bath 10 filled with sulfuric acid; the steel sheet was pickleed by the sulfuric acid in the acid pickleing treatment bath 10; the steel sheet was then fed out of the acid pickleing treatment bath 10 and fed into the acid pickleing liquid rinsing treatment bath 20 filled with water by the carrier roll 63; and the steel sheet was washed with the water in the acid pickleing liquid rinsing treatment bath 20. Subsequently, the steel sheet was fed by the carrier roll 65 (first roll) into the first electrolytic treatment bath 30, in which the cathode electrolytic treatment was performed to form metal-oxygen compound layers on the steel sheet due to actions of the anodes 80 a, 80 b, 80 c and 80 d. During this operation, the steel sheet was carried by each carrier roll and energized by the carrier roll 65 (first roll) connected electrically to a power source.

The cathode electrolytic treatment in the first electrolytic treatment bath 30 was performed to form the metal-oxygen compound layers on the steel sheet under the conditions of: a manufacturing line speed (feeding speed of steel sheet) of 10 m/min; a cycle number of 2; an energizing time for 1 cycle of 1.2 seconds; a total quantity of electricity flowing through the steel sheet of 10 C/dm²; and a magnitude of the current to the carrier roll 65 (first roll) of 70 A, while using a pump and a treatment liquid bath (not shown) provided outside the first electrolytic treatment bath 30 to circulate 1,500 L of the electrolytic treatment liquid 31 between the treatment liquid bath and the first electrolytic treatment bath 30 of a volume of 250 L. The cycle number refers to the number of times to perform electrolytic treatment to the steel sheet. (In the present example, the cycle number is 2 because the electrolytic treatment is performed two times when each side passes across the anode 80 a or 80 b and the anode 80 c or 80 d.)

Composition of electrolytic treatment liquid: Aqueous solution of a Zr concentration of 6,000 weight ppm and a F concentration of 7,000 weight ppm obtained by dissolving zirconium ammonium fluoride as a Zr compound into water

-   -   pH of electrolytic treatment liquid: 3.0     -   Temperature of electrolytic treatment liquid: 40° C.

After the metal-oxygen compound layers were formed on the steel sheet by means of electrolytic treatment, the steel sheet was fed out of the first electrolytic treatment bath 30 and fed into the electrolytic liquid rinsing treatment bath 50 filled with water by the carrier roll 65 (first roll), and washed with the water in the electrolytic liquid rinsing treatment bath 50. The surface-treated steel sheet was thus obtained.

During this operation, the value of a current flowing through the carrier roll (first roll or second roll) was measured using a clamp meter. The result is listed in Table 1.

The surface-treated steel sheet thus obtained was then evaluated as below.

<Evaluation of Presence or Absence of Arc Spot>

After the surface-treated steel sheet was washed with water and dried using a hot air drier, the surfaces of the surface-treated steel sheet were visually observed, and the presence or absence of an arc spot was confirmed on the basis of the criteria below. The result is listed in Table 1.

-   -   o: No arc spot was confirmed.     -   x: An arc spot was generated.

<Measurement of Amount of Metal-Oxygen Compound Layers>

The amount of deposited Zr on the surfaces of the surface-treated steel sheet was measured using an X-ray fluorescence spectrometer (available from Rigaku Corporation, model number: ZSX100e), and the obtained amount of deposited Zr was determined as an amount or a molar amount of the metal-oxygen compound layers. Results are listed in Table 1.

<Measurement of Thickness of Metal-Oxygen Compound Layers>

The thickness of the metal-oxygen compound layers was measured by TEM observation. After carbon was deposited on a surface of the surface-treated steel sheet, about 1 μm of carbon was further deposited thereon in an FIB apparatus, and a sample was cut out by a micro-sampling method and fixed on a Cu supporting table. Thereafter, a cross-section TEM sample was prepared by an FIB process and TEM observation was performed. The thickness of the layer was measured at each of three points, and an average value was determined as the thickness.

<Measurement of Electrical Resistance Value of Surface-Treated Steel Sheet>

Four-terminal sensing method was performed for a surface of the surface-treated steel sheet using an electrical contact simulator (available from Yamasaki Seiki Co., Ltd., model number: CSR-1) to measure an electrical resistance value five times when the terminals were in contact with the surface with a load of 100 g, and an average value was calculated using three measurements excluding the maxim and minimum values. The result is listed in Table 1.

<Measurement of Amount of Metal-Oxygen Compound Deposited on Carrier Roll>

For the carrier roll 67 (second roll) used when preparing the surface-treated steel sheet, the weight of deposited substance (oxygen compound of Zr) attached to the roll surface was measured and evaluated on the basis of the criteria below. The weight of deposited substance is converted to a weight per 1,000 cm² of the surface area of the roll, and the sampling method of the deposited substance is not particularly limited. The result is listed in Table 1.

-   -   o: The weight of deposited substance was less than 0.5 g.     -   Δ: The weight of deposited substance was 0.5 g or more and less         than 1 g.     -   x: The weight of deposited substance was 1 g or more.

Examples 2 and 3

Surface-treated steel sheets were obtained and evaluated in the same manner as in Example 1 except that the conditions of the total quantity of electricity flowing through the steel sheet and the magnitude of current to the carrier roll 65 (first roll) were changed to those listed in Table 1 when performing the electrolytic treatment for the steel sheet. Results are listed in Table 1.

Example 4

In Example 4, the surface treatment apparatus 100 illustrated in FIG. 1 and FIG. 2 was used to prepare a surface-treated steel sheet. In Example 4, the same electrolytic treatment liquid as in Example 1 was used as each of the electrolytic treatment liquids 31 and 41 in the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40. In Examples 4 to 6, the electrolytic treatment was also performed while circulating the electrolytic treatment liquids 31 and 41.

The electrolytic treatment in Example 4 was performed in the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 to form the metal-oxygen compound layers on the steel sheet under the conditions of: a manufacturing line speed (feeding speed of steel sheet) of 10 m/min; a cycle number of 4; an energizing time for 1 cycle of 1.2 seconds; a total quantity of electricity flowing through the steel sheet of 17 C/dm²; and a magnitude of the current to the carrier roll 65 (first roll) of 110 A, while carrying the steel sheet by each carrier roll. In Example 4, the cycle number, which is the number of times to perform electrolytic treatment to the steel sheet using anodes, was four. (This is because, in the present example, the electrolytic treatment is performed four times in total by four sets of anodes, i.e., the anodes 80 a and 80 b, anodes 80 c and 80 d, anodes 80 e and 80 f, and anodes 80 g and 80 h.)

After the metal-oxygen compound layers were formed on the steel sheet by means of electrolytic treatment, the steel sheet was washed with water in the electrolytic liquid rinsing treatment bath 50, and the surface-treated steel sheet was thus obtained. Evaluations were performed in the same manner as in Example 1. In Example 4, the measurement of the amount of metal-oxygen compound deposited on the carrier rolls was performed by measuring the total of weight values of the deposited substance (oxygen compound of Zr) on the surfaces of two carrier rolls, i.e., the carrier roll 67 (second roll) and the carrier roll 69 (third roll). (The same applies to Examples 5 and 6 described below). Results are listed in Table 1.

Examples 5 and 6

Surface-treated steel sheets were obtained and evaluated in the same manner as in Example 4 except that the conditions of the total quantity of electricity flowing through the steel sheet and the magnitude of current to the carrier roll 65 (first roll) were changed to those listed in Table 1 when performing the electrolytic treatment for the steel sheet. Results are listed in Table 1.

Example 7

A cold-rolled steel sheet (thickness of 0.2 mm and width of 1,000 mm) was prepared as a raw sheet. The prepared steel sheet was electrolytically degreased and then washed with water, and thereafter the surface treatment apparatus 100 illustrated in FIG. 6 was used to perform cathode electrolytic treatment. The surface-treated steel sheet was obtained and evaluated in the same manner as in Example 1 except that the liquid volume of the electrolytic treatment liquid 31 was 8,000 L, the volume of the first electrolytic treatment bath 30 was 2,500 L, and the conditions were changed to: a manufacturing line speed (feeding speed of steel sheet) of 150 m/min; a cycle number of 2; an energizing time for 1 cycle of 0.6 seconds; a total quantity of electricity flowing through the steel sheet of 9 C/dm²; and a magnitude of the current to the carrier roll 65 (first roll) of 4,760 A. Results are listed in Table 1.

Example 8

In Example 8, the surface-treated steel sheet was prepared using a surface treatment apparatus 100 in which the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 in the surface treatment apparatus 100 illustrated in FIG. 1 were substituted with those illustrated in FIG. 4. The surface-treated steel sheet was obtained and evaluated in the same manner as in Example 7 except that the liquid volume of the electrolytic treatment liquid 31, 41 was 8,000 L, the volume of each of the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 was 2,500 L, the conditions were changed to: a manufacturing line speed (feeding speed of steel sheet) of 150 m/min; a cycle number of 4; an energizing time for 1 cycle of 0.6 seconds; and a total quantity of electricity flowing through the steel sheet of 19 C/dm², and the carrier rolls 65 and 67 a (first and second rolls) were used as energized conductor rolls while the carrier roll 69 (third roll) was used as a non-energized roll. Results are listed in Table 1.

<Comparative Example 1>

In Comparative Example 1, the same cold-rolled steel sheet (thickness of 0.2 mm and width of 200 mm) as in Example 1 was electrolytically degreased and then washed with water, and the surface treatment was performed using the apparatus as below. That is, the surface-treated steel sheet was prepared using a surface treatment apparatus 100 in which the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 in the surface treatment apparatus 100 illustrated in FIG. 1 were substituted with those illustrated in FIG. 3. In the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 illustrated in FIG. 3, all of the carrier rolls 65, 67 a and 69 a are used as energized conductor rolls. Also in Comparative Example 1, the electrolytic treatment was performed while circulating the electrolytic treatment liquids 31 and 41 respectively in the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 in the same manner as in the above-described Example 4.

The electrolytic treatment in Comparative Example 1 was performed in the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 to form the metal-oxygen compound layers on the steel sheet under the conditions of: a manufacturing line speed (feeding speed of steel sheet) of 20 m/min; a cycle number of 4; an energizing time for 1 cycle of 0.6 seconds; a total quantity of electricity flowing through the steel sheet of 8.6 C/dm²; and magnitudes of the currents to the carrier rolls 65, 67 a and 69 a (first to third rolls) respectively of 68 A, 27 A and 20 A, while carrying the steel sheet by each carrier roll.

After the metal-oxygen compound layers were formed on the steel sheet by means of electrolytic treatment, the steel sheet was washed with water in the electrolytic liquid rinsing treatment bath 50, and the surface-treated steel sheet was thus obtained. Evaluations were performed in the same manner as in Example 1. Results are listed in Table 1. In Comparative Example 1, the measurement of the amount of metal-oxygen compound deposited on the carrier rolls was performed by measuring the total of weight values of the deposited substance (oxygen compound of Zr) on the surfaces of two carrier rolls, i.e., the carrier roll 67 a (second roll) and the carrier roll 69 a (third roll). The result is listed in Table 1.

<Comparative Examples 2 to 4>

Surface-treated steel sheets were obtained and evaluated in the same manner as in Comparative Example 1 except that the conditions of the total quantity of electricity flowing through the steel sheet and the magnitudes of currents to the carrier rolls 65, 67 a and 69 a (first to third rolls) were changed to those listed in Table 1 when performing the electrolytic treatment for the steel sheet. Results are listed in Table 1.

<Comparative Example 5>

The same cold-rolled steel sheet (thickness of 0.2 mm and width of 1,000 mm) as in Example 7 was electrolytically degreased and then washed with water, and the surface treatment was performed using the apparatus as below. That is, the surface-treated steel sheet was prepared using a surface treatment apparatus 100 in which the part illustrated in FIG. 2 of the surface treatment apparatus 100 illustrated in FIG. 1 was changed to that in FIG. 3. The surface-treated steel sheet was obtained and evaluated in the same manner as in Comparative Example 1 except that the liquid volume of the electrolytic treatment liquid 31, 41 was 8,000 L, the volume of each of the first electrolytic treatment bath 30 and the second electrolytic treatment bath 40 was 2,500 L, the conditions were changed to: a manufacturing line speed (feeding speed of steel sheet) of 150 m/min; and a total quantity of electricity flowing through the steel sheet of 14 C/dm², and the carrier rolls 65, 67 a and 69 a (first to third rolls) were used as energized conductor rolls. Results are listed in Table 1.

[Table 1]

In Table 1, electrical resistance values represented by “>20” show that the electrical resistance value is a value larger than 20 Ω.I

As listed in Table 1, in Examples 1 to 3 and 7, the carrier roll 65 (first roll) was used as an energized conductor roll while the carrier roll 67 (second roll) was used as a non-energized roll, and no arc spot was confirmed on the surface of the surface-treated steel sheet. Furthermore, when the surface-treated steel sheet was produced, attachment of an oxygen compound to the carrier rolls was effectively prevented, and the surface-treated steel sheet was able to be well produced. Likewise, as listed in Table 1, in Examples 4 to 6, the carrier roll 65 (first roll) was used as an energized conductor roll while the carrier rolls 67 and 69 (second and third rolls) were used as non-energized rolls, and no arc spot was confirmed on the surface of the surface-treated steel sheet. Furthermore, when the surface-treated steel sheet was produced, attachment of an oxygen compound to the carrier rolls was effectively prevented, and the surface-treated steel sheet was able to be well produced.

In Example 8, the carrier rolls 65 and 67 a (first and second rolls) were used as energized conductor rolls while the carrier roll 69 (third roll) was used as a non-energized roll, and the magnitudes of currents flowing through the first and second rolls were adjusted, thereby no arc spot was confirmed on the surface of the surface-treated steel sheet. Furthermore, when the surface-treated steel sheet was produced, attachment of an oxygen compound to the carrier rolls was effectively prevented, and the surface-treated steel sheet was able to be well produced.

On the other hand, also as listed in Table 1, in Comparative Examples 1 to 5, all of the carrier rolls (first to third rolls) were used as energized conductor rolls, and an arc spot was generated on the surface-treated steel sheet in each comparative example. Furthermore, when the surface-treated steel sheet was produced, an oxygen compound was attached to the carrier rolls, and the surface-treated steel sheet was not able to be well produced.

Referring next to Examples 9 to 11 and Comparative Examples 6 to 9 as described below, experimental examples are presented in which metal cans were prepared and filled with contents, and the resistance to the contents of the metal cans (corrosion resistance) was evaluated.

Example 9 1. Preparation of Surface-Treated Steel Sheet

A surface-treated steel sheet was prepared through the same method as in Example 4 except that a cold-rolled steel sheet of a thickness of 0.225 mm and a thermal refining degree of T3 was used as the steel sheet. Presence or absence of an arc spot was evaluated in the same manner as in Example 1. Results are listed in Table 2.

2. Preparation of Resin-Coated Surface-Treated Steel Sheet

The obtained surface-treated steel sheet was preliminarily heated to 250° C., and one surface of the metal sheet to be located inside the cans and the other surface to be located outside the cans were laminated, by thermal compression bond via lamination rolls, with a stretched copolymer layer (thickness of 19 μm) of polyethylene terephthalate/isophthalate containing 11 mol % of an isophthalic acid constituent and a stretched white copolymer layer (thickness of 13 μm) of polyethylene terephthalate/isophthalate containing 12 mol % of an isophthalic acid constituent, respectively. The laminate was immediately cooled with water, and a resin-coated surface-treated steel sheet was thus obtained.

3. Preparation of metal cans

After performing electrostatic oiling of paraffin wax to both surfaces of the obtained resin-coated surface-treated steel sheet, the steel sheet was punched out into circular shapes of a diameter of 143 mm, and drawn cups of a diameter of 91 mm and a height of 36 mm were prepared in accordance with an ordinary method. The prepared drawn cups were provided for formation of metal cans (seamless cans for 200 g contents) to be described later. Thereafter, the drawn cups were formed into cups of a smaller diameter and a larger height by performing a simultaneous drawing and ironing process two times for the drawn cups. Properties of the cups thus obtained were as follows:

-   -   a cup diameter of 52.0 mm;     -   a cup height of 111.7 mm; and     -   a thickness of the can wall part to the original sheet thickness         of −30%.

After doming formation, the cups were heated by heat treatment of 220° C. for 60 seconds in order to release strains in the resin layers, and subsequently worked into metal cans (seamless cans for 200 g contents) through: a trimming process for opening end portions; curved surface printing; a neck-in process to a diameter of 50.8 mm; and a flanging process.

4. Evaluation of Contents Filling (Packing Test (Dent Resistance)) Each of the obtained metal cans was filled with 185 ml of coffee (product name of

Blendy, bottled coffee, low-sugar, available from AJINOMOTO GENERAL FOODS, INC.) and seamed with a lid in accordance with an ordinary method, and retort treatment was carried out at 123° C. for 20 minutes. The metal can was stored at a room temperature for one day with the lid located at the upper side. Thereafter, the metal can was laid in a stationary manner, and a 1-kg weight having a spherical surface of a diameter of 52.0 mm was drop from a height of 40 mm to the side wall of the can body at a lower part so that the spherical surface would hit against the can thereby to give impact to deform the can body. After the metal can was stored at 37° C. for three months with the lid located at the upper side, the seamed part was cut using a can opener to separate the lid from the can body, and the corrosion state of the deformed part of the can inner surface was then observed using a microscope and evaluated.

Evaluation criteria were as follows. When corrosion due to blister did not occur in all of 50 cans, the evaluation result is denoted by “o.” On the other hand, when corrosion due to blister did occur in at least one can of 50 cans, the evaluation result is denoted by “x.” The evaluation result is listed in Table 2.

Examples 10 and 11

Surface-treated steel sheets were prepared through the same methods as in Examples 5 and 6 except that cold-rolled steel sheets of a thickness of 0.225 mm and a thermal refining degree of T3 were used as the steel sheets. Note that the surface-treated steel sheet prepared through the method as in Example 5 corresponds to Example 10 while the surface-treated steel sheet prepared through the method as in Example 6 corresponds to Example 11. For the obtained surface-treated steel sheets, presence or absence of an arc spot was then evaluated in the same manner as in Example 1. Thereafter, metal cans were prepared and evaluation of contents filling was performed in the same manner as in Example 9. Results are listed in Table 2.

<Comparative Examples 6 to 9>

Surface-treated steel sheets were prepared through the same methods as in Comparative Examples 1 to 4 except that cold-rolled steel sheets of a thickness of 0.225 mm and a thermal refining degree of T3 were used as the steel sheets. Note that the surface-treated steel sheet prepared through the method as in Comparative Example 1 corresponds to Comparative Example 6, the surface-treated steel sheet prepared through the method as in Comparative Example 2 corresponds to Comparative Example 7, the surface-treated steel sheet prepared through the method as in Comparative Example 3 corresponds to Comparative Example 8, and the surface-treated steel sheet prepared through the method as in Comparative Example 4 corresponds to Comparative Example 9. For the obtained surface-treated steel sheets, presence or absence of an arc spot was then evaluated in the same manner as in Example 1. Results are listed in Table 2. Note that preparation of metal cans and evaluation of contents filling were not performed for the surface-treated steel sheets prepared in Comparative Examples 6 to 9 because the presence of an arc spot was confirmed as listed in Table 2 and it was able to be determined that the resistance to the contents of cans (corrosion resistance) was poor even without performing the above-described evaluation of contents filling.

[Table 2]

In Examples 9 to 11, the carrier roll 65 (first roll) was used as an energized conductor roll while the carrier rolls 67 and 69 (second and third rolls) were used as non-energized rolls. As listed in Table 2, in all of these examples, no arc spot was confirmed on the surface of the surface-treated steel sheet, and the surface-treated steel sheet was able to be well produced.

Furthermore, the metal cans prepared by working these surface-treated steel sheets were excellent in the resistance to the contents of cans (corrosion resistance) even after the metal cans were filled with the contents, retort treatment was then performed, external forces were applied to deform the metal cans, and the metal cans were further stored for a period of time.

On the other hand, in Comparative Examples 6 to 9, all of the carrier rolls (first to third rolls) were used as energized conductor rolls. As listed in Table 2, in all of these comparative examples, an arc spot was generated on the surface-treated steel sheet, and the surface-treated steel sheet was not able to be well produced.

DESCRIPTION OF REFERENCE NUMERALS

-   1 . . . Base material -   100 . . . Surface treatment apparatus -   10 . . . Acid pickleing treatment bath -   20 . . . Acid pickleing liquid rinsing treatment bath -   30 . . . First electrolytic treatment bath -   31 . . . Electrolytic treatment liquid -   40 . . . Second electrolytic treatment bath -   41 . . . Electrolytic treatment liquid -   50 . . . Electrolytic liquid rinsing treatment bath -   61, 63, 65, 67, 67 a, 69, 69 a, 71 . . . Carrier roll -   62, 64, 66, 68, 70 . . . Sink roll 80 a, 80 b, 80 c, 80 d, 80 e, 80     f, 80 g, 80 h . . . Anode -   90 . . . Rectifier 

1. A method of producing a surface-treated steel sheet, the method comprising performing electrolytic treatment using an electrolytic treatment apparatus thereby to form a layer including a metal-oxygen compound on a surface of a steel sheet, the electrolytic treatment apparatus having electrolytic treatment baths each comprising a treatment liquid and at least one electrode, the treatment liquid containing metal ions, the electrolytic treatment being performed in each of the electrolytic treatment baths by continuously feeding the steel sheet into each of the electrolytic treatment baths and flowing a direct current between the steel sheet and the electrode, wherein: the steel sheet is continuously fed by rolls into each of the electrolytic treatment baths, the rolls comprising a roll for feeding the steel sheet into the electrolytic treatment bath and a roll for feeding the steel sheet out of the electrolytic treatment bath, both of the rolls being provided for each of the electrolytic treatment baths; the rolls provided in the electrolytic treatment apparatus comprise an energized roll that is electrically connected to a power source for flowing a direct current through the steel sheet and a non-energized roll that is not connected to a power source; and the energized roll is arranged such that, once the layer has been formed by electrolytic treatment on a surface of the steel sheet to be in contact with the rolls, the layer and the energized roll are not in contact with each other so that an arc spot is prevented from occurring.
 2. The method of producing a surface-treated steel sheet according to claim 1, wherein, among the rolls provided in the electrolytic treatment apparatus, a roll for feeding the steel sheet into an electrolytic treatment bath that performs electrolytic treatment first for the surface of the steel sheet to be in contact with the rolls is the energized roll that is electrically connected to the power source for flowing the direct current through the steel sheet.
 3. A method of producing a surface-treated steel sheet, the method comprising performing electrolytic treatment using an electrolytic treatment apparatus thereby to form a layer including a metal-oxygen compound on a surface of a steel sheet, the electrolytic treatment apparatus having electrolytic treatment baths each comprising a treatment liquid and at least one electrode, the treatment liquid containing metal ions, the electrolytic treatment being performed in each of the electrolytic treatment baths by continuously feeding the steel sheet into each of the electrolytic treatment baths and flowing a direct current between the steel sheet and the electrode, wherein: the steel sheet is continuously fed by rolls into each of the electrolytic treatment baths, the rolls comprising a roll for feeding the steel sheet into the electrolytic treatment bath and a roll for feeding the steel sheet out of the electrolytic treatment bath, both of the rolls being provided for each of the electrolytic treatment baths; the rolls provided in the electrolytic treatment apparatus comprise at least one energized roll that is electrically connected to a power source for flowing a direct current through the steel sheet and a non-energized roll that is not connected to a power source; and by adjusting combination of a resistance value of the layer on a surface of the steel sheet to be in contact with the at least one energized roll, a voltage applied to the at least one energized roll, and the number of the at least one energized roll to be provided, an arc spot is prevented from occurring when the steel sheet and the at least one energized roll are in contact with each other.
 4. A method of producing a surface-treated steel sheet, the method comprising performing electrolytic treatment in an electrolytic treatment bath thereby to form a layer including a metal-oxygen compound on a surface of a steel sheet, the electrolytic treatment bath comprising a treatment liquid and at least one electrode, the treatment liquid containing metal ions, the electrolytic treatment being performed in the electrolytic treatment bath by continuously feeding the steel sheet into the electrolytic treatment bath and flowing a direct current between the steel sheet and the electrode, wherein: the steel sheet is continuously fed into the electrolytic treatment bath by a first roll for feeding the steel sheet into the electrolytic treatment bath and a second roll for feeding the steel sheet out of the electrolytic treatment bath; the first roll is an energized roll that is electrically connected to a power source for flowing a direct current through the steel sheet; and the second roll is a non-energized roll that is not connected to a power source.
 5. The method of producing a surface-treated steel sheet according to claim 1 wherein the treatment liquid contains ions of at least one kind of metal selected from Zr, Al and Ti.
 6. The method of producing a surface-treated steel sheet according to claim 1, wherein pH of the treatment liquid is 2 to
 5. 7. The method of producing a surface-treated steel sheet according to claim 1, wherein a surface of the layer formed on the surface of the steel sheet has an electrical resistance value of 0.1 Ω or more.
 8. The method of producing a surface-treated steel sheet according to claim 1, wherein a molar amount of metal m the layer formed on the surface of the steel sheet is 0.5 mmol/m² or more.
 9. The method of producing a surface-treated steel sheet according to claim 1, wherein the layer formed on the surface of the steel sheet has a thickness of 15 nm or more.
 10. A surface-treated steel sheet for metal cans produced using the method of producing a surface-treated steel sheet according to claim
 1. 